The medical profession often needs to have an objective assessment of the health of the tissue of a patient. The patient may have suffered tissue damage as a result of malignancy, accidental or deliberate trauma such as damage incurred during a surgical operation. Tissue damage may also result from some other more persistent irritation such as being confined to bed which can lead to bed sores. It is valuable for a medical practitioner to be able to tell in advance the condition of certain tissue so he can determine the type of treatment that would benefit the patient.
It is common practice to measure an electrical property of tissue using a probe to determine its type and health. Different types of tissue have different electrical resistances. For example, malignant tumors have lower electrical resistance than healthy tissue.
Current methods of electrical tissue recognition, however, employ only one type of electrical measurement to characterize the tissue under examination, generally leading to ambiguous diagnoses of the tissue type. These measurements have included the conductivity or impedance at various frequencies using two, three and four terminals. A variety of electrode types and spacings have been employed to achieve control over the depth of penetration of the measuring current. Ollmar in U.S. Pat. No. 5,353,802 claims to have been able to control the depth of the measurement with three electrodes.
As early as 1918 Clowes (J. Proc. Soc. Exp. Biol. Med., 15, 107 (1918))showed that malignant tumors have lower electrical resistance than does healthy tissue. Schwan (Adv. Biol. Med. Phys., 5, 147-209,1957)and Webster (Ed. Electrical Impedance Tomography, Bristol, England, Adam Hilger 1990) have published extensively on the subject of tissue impedance measurement.
Machida et al in U.S. Pat. No. 4,537,203 have described an abnormal cell detecting device which employs multiple electrodes and two frequencies. The electrical property measured in this case is conductivity only. By making a number of measurements over a breast they claim to be able to detect the presence of a cancerous lesion within the breast. Ratios of conductivities are calculated and are displayed as a two dimensional array for interpretation by the operator. Machida employs the determination of only one electrical property, conductivity.
Vrana et al in U.S. Pat. No. 4,038,975 have described a circuit for measuring the electrical impedance of tissue in terms of its real and imaginary terms. From these figures they diagnose the presence of abnormalities in the tissue. They employ only one method of stimulation to make the measurement.
Juncosa et al in U.S. Pat. No. 4,729,385 have detailed an electrode suitable for the determination of the electrical impedance of cavity walls in, for example, blood vessels.
Following on the pioneering work of Langman and Burr (Amer. J. Obst. Gynec., 57, 274 (1949)) potential differences have been suggested as a means to detect skin cancer by Melczer and Kiss (Nature 179, 1177 (1957)) and by other investigators. Potential differences have been used by Faupel et al in U.S. Pat. Nos. 4,955,383, 5,217,014, 5,320,101 etc. to detect areas of neoplasm in the breast. That inventor uses multiple electrodes but again detects only one electrical property, potential differences. Melczer has published extensively on this subject and on skin resistance measurements for the detection of abnormalities.
Potential difference measurements are commonly used for both heart and brain function monitoring. Cudahy et al in U.S. Pat. No. 5,184,620 have proposed the use of multiple electrodes for signal detection and injection which are automatically selected for optimal effectiveness on the basis of the measured signals.
Honma et al in U.S. Pat. No. 4,966,158 have patented a switched electrode system for skin moisture measurement based on low frequency electrical conductivity.
Eggers et al in U.S. Pat. No. 5,630,426 have proposed using two electrodes on a probe, one of which detects the presence of an abnormality and the second applies energy to treat the abnormality. Only one form of electrical detection is used to arrive at the decision to treat the area being examined.
Electrical tomography has also been proposed by Brown et al in U.S. Pat. Nos. 4,617,939, 5,184,624 etc. for the delineation of objects within a subject. These devices use a multiplicity of electrodes to map out the conductivity patterns within the tissue of the subject. Once again only conductivity is measured.
The dielectric constant of breast tumor tissue was shown by Fricke and Morse (J. Cancer Res., 10, 340-376, 1926) to be distinctly different to that of normal breast tissue. These measurements have to be done in vitro so are not relevant to the detection of abnormalities during patient screening.
Frei et al in U.S. Pat. No. 4,291,708 describe an apparatus for the detection of breast cancer in vivo by measuring the dielectric properties of the breast using a plurality of electrodes positioned above and below the breast. Bridges in U.S. Pat. No. 5,704,355 has proposed an improvement on this concept by focusing the applied electromagnetic energy into a small area which is scanned over the breast.
Sets of four microelectrodes have been used by Kottra and Fromter (European Journal of Physiology 402: 409-420, 1984) to record the resistance across and between cells by measuring electrical impedance of cell monolayers.
Thus, it is desirable to have a system that employs more than one form of electrical property determination on the same area of tissue for a more accurate diagnosis of tissue type and condition.